In general, monolithic or multi layer type ceramic capacitors are widely used in electronic circuits of various electronic devices such as, for example, electronic computers, communications devices, television receiving sets, electronic timepieces, radio receivers and the like because of their large capacitance-size ratio and high reliability. As a dielectric material for such monolithic capacitors, there have widely been used dielectric ceramics of a barium titanate system.
One of the disadvantages of the barium titanate dielectric ceramics is that they are deoxidized when fired under the low partial pressure of oxygen, resulting in considerable decrease in specific resistance. For this reason, the barium titanate dielectric ceramics must be fired in an atmosphere with a high partial pressure of oxygen such as, for example, in air. In addition, these dielectric ceramics have a high sintering temperature of 1300.degree. to 1400.degree. C.
When producing monolithic ceramic capacitors with such barium titanate dielectric ceramics, a material for internal electrodes of the capacitors are necessarily required to meet the following conditions: Firstly, it does not melt or evaporate at the sintering temperature of the ceramic dielectrics. Secondly, it does not oxidize and does not react with dielectrics in the sintering atmosphere. These requirements are fully met by a noble metal such as, for example, platinum or palladium. The use of such noble metals however prevents reduction of the production cost.
To solve these problems, there have been proposed various non-reducible dielectric ceramics such as, for example, calcium-zirconium-modified barium titanate, in Japanese patent publication No. 56-46641 (corresponding to U.S. Pat. No. 4,115,493) and Japanese patent laid-open No. 61-19005. These non-reducible dielectric ceramics are not reduced even when fired in a reducing atmosphere, thus making it possible to use base metals as a material for internal electrodes of monolithic ceramic capacitors.
U.S. Pat. No. 4,115,493 discloses a method for producing non-reducible dielectric ceramics consisting essentially of a solid solution expressed by the formula: EQU {(Ba.sub.l-x Ca.sub.x).sub.O}m (Ti.sub.l-y Zr.sub.y)O.sub.3
wherein the subscripts x, y and m take values within the following respective ranges: 1.005.ltoreq.m.ltoreq.1.03, 0.02 .ltoreq.x.ltoreq.0.22, 0&lt;y&lt;0.20. This method comprises the steps of mixing all the raw materials, calcining the resultant mixture in air at a temperature of 1100.degree. to 1150.degree. C., preparing dielectric ceramic green sheets with resultant calcined powder, applying conductive paste for internal electrodes to the green sheets, stacking and pressing the resultant green sheets to form a monolithic multi-layer unit, and then firing the stack at 1300.degree. to 1400 .degree. C. in a reducing atmosphere. A similar method is disclosed in Japanese patent laid-open No. 61-19005.
However, the dielectric ceramics produced by such a method have a disadvantage that the change rate of dielectric constant on temperature is large. This results from the fact that the ceramic composition forms an uniform solid solution during sintering and has a single curie point.